This invention concerns a test instrument in general. More specifically, it relates to a low voltage AC ohmeter which is particularly applicable to testing insulated pipe flanges.
Insulated pipe flanges are required in many places where the handling of flammable substances are involved. The flanges with insulation perform a safety function by isolating ship and shore power systems at dock side. Similarly, pipelines are isolated from pump stations. Also, in refinery systems and in other places where flammable substances are being transported, isolations are called for. In such situations, there are safety regulations and codes which require the use of insulated flanges and which define the minimum allowable resistance across them. The regulations also specify the interval at which a flange must be tested.
In many of the foregoing installations, the nature of the fluids handled and the possibility of the accumulation of corrosion products, water, and debris across the flange insulator render it unacceptable to use a DC ohmeter. That is because polarization of such accumulation at the path being measured, can invalidate the information which is obtained. Furthermore, a standard so-called megger (AC ohmeter) will apply sufficient voltage across the flange insulator in the presence of flammables to present a possible fire hazard, and consequently is not suitable.
There are U.S. Pat. No. 3,987,390 dated Oct. 19, 1976, and U.S. Pat. No. 4,266,187 dated May 5, 1981 which are known to the applicant. However, the first of these makes use of a radio frequency signal that is applied to pipe flanges for making a resistance measurement. And, the half wave rectification detector and filter circuit that is employed uses a conventional rectifier so that the amplitude of the voltage employed must be sufficient to overcome the threshold value of such rectifier. Consequently, the safety factor for flammable substances is not provided.
The second of the foregoing patents discloses a test instrument that is particularly applicable to testing a corrosion inhibiting film. Consequently, it lacks applicability to insulated flange testing in accordance with the applicant's invention.
It is an object of this invention to provide a self-contained, portable, solid state, low voltage, AC ohmeter that is particularly applicable to testing insulated pipe flanges with safety requirements.
Another object of the invention is to provide a low voltage, AC ohmeter that includes high-low limits with visual readout so that the range of resistance value may be indicated to show the state of a flange insulator and indicate when replacement is called for, even before failure.